In the 1970s, the Narrabri intensity interferometer was used to measure 32 stellar diameters; some as small as 0.4 mas. The interferometer consisted of a pair of 6.5 m telescopes (30 m~2) with relatively crude optics, similar to those currently in use as Atmospheric Cerenkov Telescopes (ACTs). We explore the possibility of implementing a modern intensity interferometer on an ACT array. Developments in fast digital signal processing technology now make such a system relatively easy to implement, and provide improved sensitivity. Allowing measurements at short wavelengths ( < 400 nm), with long baselines ( > 100 m), which are still challenging for Michelson interferometers, present ACT arrays could be used to probe angular structures as small as ～ 0.2 mas and smaller, with large array projects already being discussed. This would provide measurements of stellar diameters, binary systems, circumstellar environments, and possibly stellar surface features. ACT arrays could be used as intensity interferometers during bright moon periods, providing valuable scientific output for little expense and no impact on the γ-ray observing schedule.
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